Matter files: portal for clients, workbench for staff

Every Project also has a matter file surface. Clients see it as Documents, Engagements, Invoices, and other plain-English portal views. If a person is added to a Project, Navigator can show them the client-facing files for that matter; if they are not added, those files do not exist from their portal's point of view.

Staff users have the firm workbench. Assigned staff can work the Project through /staff, and staff-tier users with Project access can use the Project's Git repository for editor-style legal work — clone the matter, edit files, push back to the single main history — through navigator auth login and navigator git clone <project-code>. The CLI creates ~/Projects when needed and checks the matter out at ~/Projects/<project-code>, then derives short-lived Git credentials from the active Navigator login. Admins are staff-tier users with installation-wide authority, so they can reach the same firm workbench without per-Project assignment. Clients never need that layer and never see clone URLs, branch names, commit SHAs, Git LFS pointers, or the private GCS bucket behind the files.